A fuel cell, which generates water (H2O) by binding of hydrogen (H2) with oxygen (O2), and draws out electrons (e−), recently receives attention as clean energy because only water is generated by the reaction. However, a hydrogen fuel cell which is a fuel cell using hydrogen is costly because a noble metal such as platinum is used as an electrode catalyst. It is also necessary to separately purify and modify fuel hydrogen because it is not available in the surroundings.
Under these circumstances, as a fuel cell capable of generating electricity from organic matters such as hydrocarbons or amino acids in the surroundings by using microorganisms as an electrode catalyst, a microbial (bio) fuel cell receives attention as low-cost clean energy. A microbial fuel cell is generally so configured that oxygen is reduced at the cathode electrode, and an organic fuel such as hydrocarbons or amino acids is oxidized at the anode electrode. In the microbial fuel cell, the reduction of oxygen at the cathode electrode can be carried out by using an electrode material having oxygen reducing ability such as carbon besides a noble metal catalyst such as platinum. In addition, in the microbial fuel cell, oxidation of a fuel at the anode electrode is carried out by the electrode that receives electrons that are transmitted in the process in which the microorganism carried on the anode electrode metabolizes (oxidizes) the fuel.
For example, Japanese Patent Laying-Open No. 2007-324005 (PTD 1) discloses a microbial fuel cell having such a structure that an anode tank (culture tank) and a cathode tank (redox reaction tank) are separated by an insulating ion exchange membrane, the anode tank is hermetically filled with a solvent that is a mixture of a microorganism which serves as a catalyst of the anode, and an organic molecule which serves as a fuel therefor, and the cathode tank is hermetically filled with a solvent that is a mixture of molecules mediating electrons between oxygen and an electrode, and is different from that in the anode tank.
In order to configure the microbial fuel cell more simply at low cost, it is possible to configure a microbial fuel cell by embedding an electrode which serves as an anode in soil, and disposing a cathode electrode on the surface of the soil (See the publicly known information listed in “Educational Resources” in the item of “Community” on the homepage of Keego Tech (http://www.mudwatt.com/pages/educational-resources) [searched on Jun. 17, 2015] (NPD 1). For example, “Dirt=Power: An Intro to Microbial Fuel Cells” recited as “Beginner Intro to Microbial Fuel Cells.”). The cell structure like this eliminates the need of extraction of microorganisms, adjustment of an anode solution and a cathode solution, and an expensive ion exchange membrane having high molecular permeation selectivity, and thus it is possible to realize a microbial fuel cell conveniently at low cost.
However, microorganisms that are used in the anode of the microbial fuel cell as described above are often anaerobic. The term “anaerobic” used herein encompasses both “obligatory anaerobic” and “facultative anaerobic.” When power generation is conducted by disposing an electrode in soil or mud, an obligatory anaerobic microorganism cannot live in the surface part of the soil and mud where oxygen is abundant, and a facultative anaerobic microorganism cannot metabolize (oxidize) a fuel in the surface part of the soil and mud where oxygen is abundant, so that the anode is inevitably disposed in a deep position far from the surface. In addition, since oxygen exists near the anode, the performance of the cell is also deteriorated by a redox reaction at the anode. Accordingly, the distance between the anode and the cathode is inevitably increased, and this makes it difficult to thin and miniaturize the cell.